Dear Tracie,
There are many kinds of creatures that lived on Earth long ago and their relatives still exist today. We know about them from fossil records, imprints or remains that we find in rock.
Horseshoe crabs, velvet worms, and sea sponges have been found in fossils that are hundreds of millions of years old. We know that creatures like jellyfish, lampreys, and sea urchins have been around about that long, too. These fossil records can provide us with clues about bones, plants, shells, and even bacteria.
While these living things have been around a long time, there is one organism that has outlasted pretty much all others. It is so small you would likely need a microscope to see it: cyanobacteria.
While you are made up of billions of cells, a cyanobacterium is one tiny cell. Just like some other types of bacteria, cyanobacteria can come in different shapes and sizes: spirals, spheres, and rods.
We can find them living on rocks, in soil, lakes, ponds, and the ocean. They can live in hot springs and even the chilly temperatures of Antarctica.
If you’ve ever seen the surface of a pond or lake covered in bright green slime, there’s a chance you might have seen a big colony of cyanobacteria.
I learned about these long-lived bacteria from my friend Michael Berger. He teaches marine biology at Washington State University.
Berger studies barnacles, which have been on the planet for about 500 million years. He told me that cyanobacteria have been around even longer than barnacles.
One of the oldest fossil records of cyanobacteria was found in Australia. It was nearly 2.7 billion years old. For a bit of perspective, that was way before dinosaurs roamed the earth between 230 and 65 million years ago.
Even though cyanobacteria are very small, they have a really important job. You may have heard that plants go through something called photosynthesis. Cyanobacteria do this, too.
They take carbon dioxide dissolved in water, use sunlight as energy to turn that carbon dioxide into sugars and then use those sugars as their food. In the process, they give off oxygen.
Cyanobacteria were one of the very first bacteria to produce oxygen in Earth’s atmosphere—and they have been doing so ever since. That’s good news for all of us plants and animals that need oxygen.
Berger reminded me that our answer to your question about the history of life on Earth can change as we discover new things. We may not have all the answers to your question just yet, but we can rely on clues from the past to help us understand our world.
Who knows, maybe one day you can make a career out of looking for fossils and telling us about them. Or maybe you’ll defend species that live on our planet right now so we can help protect their futures.
Sincerely,
Dr. Universe